The present invention relates to a method for adhering a masking film to a wafer and an apparatus therefor, and more particularly, to a method for adhering a masking film to a silicon wafer by precisely cutting the masking film, and an apparatus therefor.
The integrated circuit manufacturing process involves steps of the crystallizing process, the pre-wafer process, the after-wafer process, the packaging process and the testing process. In the crystallizing process, the slicing, lapping, polishing or back-grinding steps are performed after crystallization of the silicon. The lapping step is aimed at removing the strained layer, which was generated in the slicing step, from the surface of the silicon crystal, and at thinning the sliced thick crystalline wafer to a predetermined thickness and finishing the sliced surface so as to have a mirror finish for electric circuit formation. Accordingly, a considerably high precision is required for surface flatness.
Recently, it has become routine to perform the thinning process after the wafer forming process, to fulfill packaging requirements and to enhance heat dissipation as well as to reduce the size of the package. In the case of CMOS devices and the like which generate less heat, the thinning of the wafer is less critical. However, in order to minimize the size of the package, the thinning step is necessary. In the process of thinning the wafer, the back side of the wafer is lapped or grinded.
Prior to the lapping stage, the overall surface of the silicon wafer where a circuit is to be formed is coated with a protective film, or a masking film is stuck to the circuit face of the silicon wafer. This operation is intended to protect the circuit face of the silicon wafer during the lapping, etching or back grinding step. This operation has been carried out manually. Specifically, a manual operation has been relied upon to cut off the outer periphery of the masking film, which extends over the silicon wafer edge, along the outer periphery of the silicon wafer. Even though this manual operation is performed by a skilled worker, it is inefficient and troublesome. To solve the problems resulting from this manual operation, a method and apparatus for automatically cutting the masking film has been disclosed in U.S. Pat. No. 4,779,497 (issued to Mishahiro Lee). In addition, a method for automatically peeling the masking film has been disclosed in Japanese Laid-open Publication No. 63-9122.
FIG. 1 schematically depicts a conventional apparatus for adhering a masking film to a silicon wafer. As shown in FIG. 1, the apparatus includes masking film tape supply roll 1, masking film tape 3, first guide roller 5, second guide roller 7, adhesive rollers 9a and 9b consisting of upper and lower rollers, wafer 11, cutting table 13, supporting rollers 15a and 15b consisting of upper and lower rollers, and masking film tape take-up roll 17. Arrow 21 denotes the cutting location of a masking film.
One process for adhering a masking film onto a wafer using the above apparatus is as follows. First, masking film tape 3 is drawn out from masking film tape supply roll 1. Then, masking film tape 3 passes through first guide roller 5 and second guide roller 7 and is transported to upper and lower adhesive rollers 9a and 9b.
One surface of masking film tape 3 which contacts wafer 11 is coated with adhesives and acquires adhesiveness. Wafer 11 is provided from the right side of adhesive rollers 9a and 9b so that the adhesive surface of masking film tape 3 and the circuit face of wafer 11 may come into contact with each other. Thus, wafer 11 and masking film tape 3 together pass through adhesive rollers 9a and 9b. Masking film tape 3 is stuck onto wafer 11 by adhesive rollers 9a and 9b, and then transported onto cutting table 13 by the driving force of masking film tape take-up roll 17. During the transportation, masking film tape 3 moves while maintaining an approximately 5 mm spacing between masking film tape 3 and cutting table 13. Masking film tape 3 stops when wafer 11 is positioned over the aperture in the center portion of cutting table 13. Supporting rollers 15a and 15b support the masking film tape, at a height roughly equivalent with that achieved by adhesive rollers 9a and 9b, so that masking film tape 3 moves horizontally while maintaining a predetermined space from cutting table 13. At the state where masking film tape 3 stops, a cover (not shown) having a vacuum chuck therein is placed on the upper portion of wafer 11, to thereby fix wafer 11 and masking film tape 3. At the fixed state of the wafer on cutting table 13, a cutter (not shown) located at the lower portion moves upward and cuts masking film tape 3 along the periphery of the wafer.
FIG. 2 is a plan view showing wafer 11 situated over cutting table 13. Here, reference numeral 13a denotes an aperture formed in the center portion of cutting table 13 so as to receive wafer 11, and reference numeral 21a denotes a cutting line which exists between aperture 13a and wafer 11. With wafer 11 placed over the center of aperture 13a of cutting table 13 as shown in FIG. 2, a cutter (not shown) provided below cutting table 13 cuts masking film tape 3 along cutting line 21a (which coincides with cutting location 21 of FIG. 1) between wafer 11 and aperture 13a, to form a wafer-shaped cut at the inner portion of masking film tape 3, thereby adhering a masking film onto the wafer. After the cutting operation, wafer 11 covered (adhered) with the masking film is transported by a vacuum chuck (not shown) for a subsequent process. In addition, the remaining masking film tape 3 which has been cut along the shape of wafer 11 and bears a wafer-shaped hole, passes through supporting rollers 15a and 15b and is wound on masking film tape take-up roll 17.
A problem with the above described method is that masking film tape 3 may be wound onto lower adhesive roller 9b instead of being passed between the rollers, due to the adhesive force of masking film tape 3. As a result, when wafer 11 is introduced to the apparatus between adhesive rollers 9a and 9b, the wafer is broken as it becomes wrapped around the lower adhesive roller. FIG. 3 illustrates this phenomenon wherein masking film tape 3 becomes wound onto lower adhesive roller 9b.
Another problem is that the masking film tape 3 sags due to the weight of silicon wafer 11, which causes masking film tape 3 to adhere to or be snagged by cutting table 13. FIG. 4 illustrates this phenomenon wherein masking film tape 3 becomes adhered to the peripheral edge of cutting table 13 due to the weight of wafer 11. Regions A and B of FIG. 4 show the portions of masking film tape 3 which adheres to cutting table 13.
Further, in the conventional method for adhering a masking film, a motor for driving masking film tape take-up roll 17 is operated without any consideration of the wound amount of the masking film tape accumulated after the cutting operation. When the wound amount of masking film tape 3 is large, the diameter of the take-up roll of tape is large and a greater tension is applied to masking film tape 3 in the lengthwise direction. In this case, since the inner portion of masking film tape 3 which is being wound onto masking film tape take-up roll 17 has a plurality of serial cuts punched according to the shape of wafer 11, the lengthwise tension cannot be uniformly applied to masking film tape 3 across its width. Due to this non-uniform tension, the location of the silicon wafer cannot be precisely controlled.
In the above case, wafer 11 is damaged by the cutter and/or the cutting of masking film tape 3 is rendered without uniformity.
Further, since the tension can be applied only at the outer portions of masking film tape 3 remaining after the cutting process, the inner portion of masking film tape 3 becomes wrinkled. FIG. 5 illustrates masking film tape 3 having a wrinkled portion 4 formed in the inner portion thereof between the wafer-shaped holes. In such a case, the wrinkled portion is not completely cut by the cutter during the cutting process, thereby inhibiting separation from the tape. Therefore, wafer 11 is dragged by masking film tape 3 which continues being wound, which results in damage to wafer 11. In FIG. 5, region C shows the portion where the incomplete cutting of masking film tape 3 occurs.